I've had 2 occasions in my life where filtering has led to drivers attempting to use the car as a block to prevent me filtering, both at very low speed and thanks to very rough old bikes, on both occasions the cars left with more dents than the bike, I am damn lucky to have come out of it unscathed. Lots of times idiots have moved to block filtering, generally I only filter in total standstill traffic, which makes some plonker moving over to block me a clear cut case of them moving to interfere with other traffic.
Mid Wales is still an awesome place to ride motorbikes. I wasn't so keen on my ex police bike covered in yellow and blue stickers done by the previous owner, the blue text actually said Peugeot but at a glance in a rear view mirror I can guess what folk assumed, almost every time I moved out to overtake the cars in front would slow down and pull over, which I found disconcerting particularly if they braked hard.